The invention relates to a motor vehicle combustion engine with exhaust-gas recirculation.
PCT publication WO 2008/102230 A1 disclose a combustion engine with exhaust gas recirculation with a particle filter and an upstream SCR catalyst arranged in the exhaust gas section. Ammonia or urea can be conveyed upstream from the SCR catalyst into the exhaust gas section as a nitrogen oxide reduction means by means of an addition device. The exhaust gas recirculation thus takes place via a low-pressure path diverging from the exhaust gas section downstream of the particle filter.
Similar combustion engines described in PCT publication WO 2008/030314 A1, and in German publication DE 10 2009 014 361 A1, which additionally possess an exhaust gas recirculation via a high-pressure path. A further SCR catalyst, arranged downstream of the particle filter, is provided in PCT publication WO 2008/030314 A1.
When there is exhaust gas recirculation via a low-pressure path, it can, however, lead to pollution of an exhaust gas cooler arranged in the low-pressure path, due to, for example, nitrogen oxide or sulphur-based compounds contained in the recirculated exhaust gas.
Exemplary embodiments of the present invention provide a motor vehicle combustion engine that enables efficient exhaust gas purification with exhaust gas recirculation with as low a level of contamination as possible.
The motor vehicle combustion engine according to the invention has an air supply system for supplying combustion air to the combustion engine and an exhaust gas section containing exhaust gases of the combustion engine, in which a particle filter and an SCR exhaust gas purification component, which is able to carry out the selective catalytic nitrogen oxide reduction by means of ammonia, are arranged. Thus, for the combustion engine according to the invention, an addition device for adding ammonia or a reduction means capable of separating ammonia is provided upstream of the SCR exhaust gas purification component. Furthermore, a first exhaust gas turbocharger is provided, the turbine of which is arranged upstream of the particle filter in the exhaust gas section. A first exhaust gas recirculation, which diverges from the exhaust gas section upstream of the turbine of the first exhaust gas turbocharger, and a second exhaust gas recirculation, which diverges from the exhaust gas section downstream of the particle filter, are provided to recirculate the exhaust gas from the exhaust gas section into the air supply system. An SCR catalyst is arranged in the second exhaust gas recirculation line.
The first exhaust gas recirculation line, which diverges upstream from the exhaust gas turbocharger turbine, is a high-pressure exhaust gas recirculation path, via which a high-pressure proportion of recirculated exhaust gas can be fed from the exhaust gas section into the air supply system. The second exhaust gas recirculation line, which diverges from the exhaust gas section downstream of the particle filter, represents a low-pressure exhaust gas recirculation path, via which a low-pressure proportion of recirculated exhaust gas can be fed into the air supply system.
It is possible to improve nitrogen oxide conversion by means of the SCR catalyst arranged in the second exhaust gas recirculation line. The SCR catalyst in the second exhaust gas recirculation line enables load relief of the SCR exhaust gas purification component in the exhaust gas section and reduces both the nitrogen oxide and ammonia content of the exhaust gas recirculated via the low-pressure path. Contamination, for example by ammonia or nitrogen oxide compound deposits in the second exhaust gas recirculation line, is thus avoided and the exhaust gas emission of the combustion engine is improved. As a consequence of the resulting nitrogen-oxide-low exhaust gas recirculation, the fuel combustion in the combustion engine is improved. The SCR catalyst in the second exhaust gas recirculation line also enables at least comparatively large particles to be absorbed, whereby a separate large particle filter can be omitted in the second exhaust gas recirculation line. In addition, the SCR catalyst produces standardized flow, whereby pressure losses via the second exhaust gas recirculation line are reduced. The SCR catalyst in the second exhaust gas recirculation line and the SCR exhaust gas purification component in the exhaust gas section thus comprise a catalytically effective material with respect to selective nitrogen oxide reduction under oxidizing conditions by means of ammonia. The SCR catalyst in the second exhaust gas recirculation line is preferably designed as a supported honeycomb body with an SCR catalyst material coating or as a fully-extruded honeycomb body, for example based on vanadium pentoxide/titanium oxide/tungsten oxide. The SCR catalyst material coating can also be applied to a particle filter structure that is suitable for filtering particles. It is particularly preferably to have a honeycomb body monolith with a zeolite coating containing copper or iron.
In accordance with one aspect of the present invention, the SCR exhaust gas purification component arranged in the exhaust gas section is designed as a catalytic coating of the particle filter and/or as a separate SCR catalyst component arranged upstream and/or downstream of the particle filter in the exhaust gas section. In an embodiment, this can be applied to the impure gas side or to the purified gas side of the filter-effective material as a catalytic coating of the particle filter. In the case of a particle filter designed as a wall-flow filter, the coating can be applied to the surface of individual or all gas inlet channels or to the surface of individual or all gas outlet channels. Thus, the coating can also be provided only in sections, preferably on an upstream sectional region of the particle filter. It is preferable for a separate SCR catalyst component to be provided in the exhaust gas section additionally or alternatively. This is preferably provided downstream of the particle filter, in particular downstream of the point of divergence of the second exhaust gas recirculation line in the exhaust gas section. However, provision can also be made for an arrangement to be directly on the inlet or outlet side of the particle filter. Provision is preferably made for the larger proportion of nitrogen oxide reduction to be carried out by the SCR exhaust gas purification component and for the smaller proportion to be carried out by the SCR catalyst arranged in the second exhaust gas recirculation line.
In a further embodiment of the invention, an oxidation catalyst in the exhaust gas section is arranged downstream of the turbine of the first exhaust gas turbocharger, and upstream of the particle filter. The oxidation catalyst enables oxidative displacement of excess hydrocarbons in the exhaust gas. Furthermore, nitrogen monoxide contained by the oxidation catalyst in the exhaust gas is at least partially oxidized into nitrogen dioxide, which can itself displace soot that is deposited in the particle filter at comparatively low temperatures (300° C. to 450° C.) by oxidation. Furthermore, an increased concentration of nitrogen dioxide enables an improved nitrogen oxide reduction process at the SCR exhaust gas purification component. For this, provision is preferably made to adjust a nitrogen dioxide proportion of nitrogen oxides contained in the exhaust gas to approximately 50% on the inlet side of the SCR exhaust gas purification component, by means of suitable adjustment of the relationship between the amount of exhaust gas that is recirculated via the low-pressure exhaust gas recirculation path and the high-pressure exhaust gas recirculation path. Moreover, by oxidizing the hydrocarbons that are additionally inserted into the exhaust gas, it is possible to raise the exhaust gas temperature, if necessary. In particular, for thermal regeneration of the particle filter by soot combustion, the exhaust gas can be enriched with hydrocarbons by after injection in the engine or by secondary fuel injection outside of the engine.
In a further embodiment of the invention, provision is made to add ammonia or the reduction means that is capable of separating ammonia downstream of the oxidation catalyst and upstream of the particle filter. Thus, on the one hand, ammonia oxidation is avoided, while on the other hand, hydrolysis of urea that is added as a reduction means, if necessary, is improved as a consequence of the flow path through the particle filter.
In a further embodiment of the invention, adjustment means for adjusting the recirculated amount of exhaust gas are provided, which comprise an adjustable restrictor element, arranged in the exhaust gas section downstream of the point of divergence of the second exhaust gas recirculation line and/or in the second exhaust gas recirculation line of the in-flow point into the air supply system thereof, and/or an adjustable restrictor element, arranged in the first exhaust gas recirculation line of the in-flow point into the air supply system thereof. It is thus also possible to adjust the relationship between the high-pressure exhaust gas recirculation rate and low-pressure exhaust gas recirculation rate over practically the entire operating region of the combustion engine, depending on need and the working point, just as a variable adjustment of the total amount of exhaust gas recirculation. If an SCR catalyst is provided in the exhaust gas section downstream of the point of divergence of the second exhaust gas recirculation line, the restrictor element is arranged in the exhaust gas section, preferably between the point of divergence and the SCR catalyst. An arrangement downstream of the SCR catalyst is also possible.
The adjustment means for adjusting the low-pressure proportion and/or the high-pressure proportion of the total amount of recirculated exhaust gas are thus also used, depending on the working point of the combustion engine, to adjust a proportion of nitrogen dioxide (NO2), which is advantageous for the catalytic reduction of nitrogen oxide of the SCR exhaust gas purification component, of the nitrogen oxides that are present in the exhaust gas on the inlet side of the SCR exhaust gas purification component. Thus, an adjustment of the low-pressure proportion of recirculated exhaust gas is performed in such a way that a proportion of NO2 results which is lower than 70%. It is particularly preferable to adjust the low-pressure proportion in such a way that the NO2 proportion of NO2 contained in the exhaust gas is approximately 50%.
In a further embodiment of the invention, a second exhaust gas turbocharger is provided, the turbine of which is arranged downstream of the turbine of the first exhaust gas turbocharger in the exhaust gas section. This enables two-tiered charging of the combustion engine with a corresponding increase in efficiency with a comparatively reduced pollutant discharge.
In a further embodiment of the invention, a circumventable charge air cooler is arranged in the air supply system to cool compressed combustion air. This enables a variable decrease in the combustion temperature of combusted fuel in the combustion chambers of the combustion engine and thus a further decrease in the pollutant emission of the combustion engine, in particular with respect to nitrogen oxide.
In a further embodiment of the invention, an exhaust gas cooler is arranged in the first exhaust gas recirculation line and/or the second exhaust gas recirculation line in order to cool the exhaust gas that is recirculated to the air supply system. This also enables a decrease in the combustion temperatures. It is thus also advantageous if a bypass line is provided in a further embodiment of the invention for the exhaust gas cooler arranged in the first exhaust gas recirculation line and/or in the second exhaust gas recirculation line.
Further advantages, features and details of the invention arise from the description of preferred exemplary embodiments below, as well as with the aid of the FIGURE. The features and feature combinations cited in the description above and the features and feature combinations cited below in the description of the figures and/or shown in the FIGURE alone cannot solely be used in each specified combination, but rather also in other combinations or individually, without exceeding the scope of the invention.